Mary Theresa doesn't appear in many major quests, but she does appear in some.

In the first quest, when a princess is going to stay over at Noble's house, he asks The Player to get an item called Expensive Furniture from Mary Theresa, because it was supposed to come earlier that day. She gives it to them, and The Player gives it to Noble.

In her third quest, Noble broke his Chairman Staff; a family heirloom that is used when hosting meetings. He asks The Player to go get glue from Mary Theresa, but since she didn't have any, The Player goes to Zoomy Lake to get some. It's on the tiny island to the left of the island path to Zoomy Village.

In her last quest, Noble needed some cork, and asked The Player if they could go to Mary Theresa to get some. Alas, like last time, Mary Theresa is out, so The Player has to go find some cork.